Mantle cell lymphoma (MCL) is the most therapy-resistant;B cell, non-Hodgkin's lymphoma (NHL-B) that currently lacks adequate therapy, and is increasing in incidence in the USA. Understanding the patho- physiology of MCL and experimental therapeutics for better MCL therapy has been hampered by the lack of a valid animal model. We have recently constructed the first transgenic MCL model that closely simulates the most therapy-resistant form of MCL, the Blastoid variant (MCL-BV). This model not only provides potential insights into the pathogenesis of MCL-BV, but also has a hierarchical tumor cell organization, with a small but distinct tumor cell subpopulation with self-renewal capabilities that resemble tumor initiating cells (TIC). The TIC population in our MCL-BV model also shows similarities to Cancer Stem Cells (CSC) that may represent the "cell of origin" in this MCL-BV model that should allow for testing various predictions of the Cancer Stem cell hypothesis, in a valid murine MCL model. Our MCL-BV model is a murine double transgenic (DTG) that was constructed by crossing IL14 x c-myc single TG mice. IL-14, a cytokine growth factor for normal B-lymphocytes, is over-expressed in NHL-B, where it may function as an oncogene in the B lymphocytic lineage, showing important B cell functional characteristics in addition to growth stimulation. DTG mice stochastically develop NHL-B resembling aggressive MCL-BV by 3-4 months (100% penetrance), allowing for sequential and defined patho-physiologic characterization of molecular and genotypic alterations in the murine B lymphoid cell compartment experimentally, from birth to lymphoma development. The DTG MCL-BV model provides multiple interesting potential insights into MCL, but our studies in this R21 proposal will focus on understanding one aspect in MCL-BV ontogeny, by identifying and characterizing a possible "cell of origin" in DTG/MCL-BV model. These CSC characteristics include tumor cell "self-renewal" characteristics and transplantabilty into immune-deficient (SCID) mice. In this proposal, we will focus primarily on: (1) characterizing the hierarchical nature of DTG tumor cell populations, identification of SP populations to support our hypothesis that our MCL model displays Tumor Initiating Cells (TIC) with Cancer Stem Cell (CSC) characteristics;and (2) further characterizing and validating that our DTG/MCL-BV model simulates anomalous cell cycle and constitutive growth/survival signaling pathway activation, characteristic of human MCL-BV. Our studies will explore how CSC-associated properties of putative MCL/ TIC in the DTG/MCL-BV model, contributes to the initiation, tumor maintenance, and possibly recurrence of the DTG lymphoma. We will also examine whether well-known human MCL characteristics, involving abnormal cell cycle regulation with cyclinD1 over-expression, and the constitutive activation of the NF-?B and pAKT growth and survival (anti-apoptotic) signaling pathways, are also activated in DTG/TIC tumor cells and/or other DTG tumor sub- populations, that may account for the clinical therapeutic resistance seen in patients with MCL, and particularly MCL-BV. PUBLIC HEALTH RELEVANCE: Mantle cell lymphoma (MCL) is the most difficult B cell Non-Hodgkin lymphoma to treat clinically, with the worst survival prognosis. Why MCL is so clinically aggressive is not known, which along with the lack of a suitable animal model has hampered development of more effective therapies. We have developed a genetically engineered model (double transgenic;DTG) of the blastoid variant of MCL (MCL-BV) that will provide both clues to the origin/causes of MCL as well as a model to test new MCL drugs, which will be developed in this grant proposal.